The electrical membrane properties of the marine (brackish-water) ciliate Paramecium calkinsi were investigated under constant-current and voltage-clamp conditions, using two intracellular microelectrodes. The action potential and membrane currents were extremely tolerant to changes in the salinity of the bathing medium. Current-voltage relationships exhibited a moderate inward-going rectification of the membrane upon hyperpolarization, and a prominent outward-going rectification upon depolarization of the membrane. Ion substitution experiments showed that the electrically excitable response is a graded Ca2+-action potential, similar to that found in freshwater ciliates.

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